Process for replenishing and correcting the electrolyte in the refining of aluminum



Juhe 10, 1941. SIEDENTQPF I 2,245,505

PROCESS FOR REPLENISHING AND CORRECTING THE ELECTROLYTE IN THE REFINING 0F ALUMINUM Filed'July 1-. 1939- Patented June 10, 1941 raocnss. roa aarmmsnmd AND coaacormo THE mcraonrram THE aurmmc or ALUMINUM Karl Sicdentopf, Bitterfeld, Germany, assignor to Compagnie de ProduitsChimiques et Electrometallurgiques Alais, Froges et Camargue, Paris, France, a corporation of France Application July 1, 1939, Serial No. 282,547 In Germany July 12, 1538 3 Claims.

The manufacture of very pure aluminum through igneous electrolysis methods-in which,

consequently, the usual aluminum of the market (raw aluminum) is freed from impurities which are alloyed therewith, in particular iron and silicon-takes place, as it is known, by the fact that a molten alloy of raw aluminum and of a heavy metal constitutes the anode and that above this anodic layer there is first disposed a layer of -molten halogenides, such as fluorides, chlorides which acts as electrolyte, and that, finally, the upper layer, which acts as cathode, consists of refined aluminum. In this last mentioned layer are immersed the carbon rods which are acting as electrodes.

In this method of refining aluminum, there is not finally produced, to be true, an electro-chemical decomposition of the electrolyte. However, in the electrolyte, losses are constantly taking place with respect to.some components thereof, either by volatilization or by separation of solid components in the form of crusts. It is therefore necessary to replenish the molten mass of the electrolyte from time to time and also to correct its composition.- It is extremely important for the refining of aluminum through the electrolytic process to find a solution of this problem which is as simple as possible and does not interfere with the working of the system.

It has already been suggested to replenish and to correct the electrolyte by melting together the components which are to be fed to the electrolyte, in suitable proportions, in a distinct tank, subjecting them to a. preliminary refining and incorporating them to the refining bath proper. The incorporation can be eifected by the fact that the cathodic layer of very pure aluminum is removed and the refined molten mass is then mixed to the without stopping electrolysis, added directly into the surface of a limited portion of the electrolyte, freed from the layer of very pure aluminum which acts as cathode.

According to a preferred embodiment of the present invention, this method is carried out by suspending a tube of graphite, the diameter of which is advantageously substantially equal to its height, in the refining bath in such manner that,

electrolyte, or otherwise by the fact that the preliminarily refined and solidified electrolyte, the specific weight of which must, in this case, be considerably higher than that of refined aluminum, is added, in the form of rough lumps, into the cathodic layer of pure aluminum, so that it sinks through 'said layer, passes into the electrolyte and melts therein.

The object of the present invention is to proextending through the cathodic mass of pure aluminum, this tube has its base immersed in the layer of electrolyte, being however out of contact with the anodic alloy which constitutes the lowermost layer. The amount of pure aluminum present inside the graphite tube is then removed, until the electrolyte comes to the surface. The components necessary for correcting the composition of the electrolyte are then added thereto, in the tube. in suitable proportions. The components dissolve relatively quickly in the electrolyte.

Other features of the present invention will result from the following detailed description of a specific embodiment thereof.

Preferred embodiments of the present invention will be hereinafter described, with reference to the accompanying drawing. given merely by way of example, and in which:

The only figure is a vertical section of a tank for the electrolytic refining of aluminum, illustrating the application of the present invention. In the apparatus il ustrated by the drawing, reference numeral (1 designates the refining tank; the lower anodic layer is designated by reference numeral b. It consists of an alloy of aluminum and a heavy metal. Above this layer is the layer of electrolyte 0, above which is the cathodic layer 11, constituted by pure aluminum. Reference numeral 1 designates the tube, extending through the cathodic layer and immersed in electrolyte c, owing to which it. is possible to replenish and to correct the composition of said electrolyte .through the addition of the necessary salts.

Because of the extreme purity of the refined aluminum produced by the refining tanks which are actually used it is necessary that the electrolyte also be perfectly pure. It is actually impossible to find industrial salts '(fiuorides, chlorides, etc.) sufiiciently pure to be used di; rectly as an electrolyte in a refining tank. These salts always contain metallic impurities (iron,

silicon, manganese, etc.) which are immediately electrolyzed in the tank and which contaminate the refined cathodic aluminum.

In order to eliminate the impurities of the --and the electrodes.

salts which are to be fed to the electrolyte, and particularly the impurities which can be removed only through an electrolytic treatment, it is advisable to leave on the inside of tube f a small amount of aluminum while the electrolyte is being-replenished. Alternately, according to the invention, a certain amount of liquid aluminum is introduced into the tube of graphite I, after addition of the complementary salts. The liquid metal is thus loaded, in a known manner, with a part of the impurities. At the same time, the molten metallic mass, floating on the top of the electrolyte present on the inside of the tube, can act as auxiliary cathode, because it is electrically connected, through the medium of the graphite tube that surrounds it, to the main cathode, consisting of pure molten aluminum and in contact with the external wall of the tube. Eventually, this electrolytic treatment of the portion of the electrolyte located on the inside of the tube is continued until the impurities are eliminated from this portion of the electrolyte.

When the melting or refining operation is finished, eventually after having previously withdrawn the auxiliary cathode of molten raw or pure aluminum, the tube of carbon is removed from the bath so that the level equilibrium is re-established in the electrolyte and the layer of pure aluminum, acting as main cathode, again covers the whole of the electrolyte.

The method above described has, over those suggested up to the present time, the advantage that it does not require the preparation of a particular tank for the preliminary refining of the electrolyte, nor does it necessitate an interruption of the working of the refining tanks,

even while the electrolyte is being replenished and has its composition rectified.

It is also possible, according to the invention,

to dispense with the gradual correction of the composition of the electrolyte, which up to now has been considered as frequently necessary, because the addustment of the composition of the electrolyte through the method above described is complete in a single operation.

The method according to the invention also eliminates the disturbances brought to the operation of the refining tank, which disturbances are due to the thorough mixing which is otherwise necessary for adding the new electrolyte to that already present in the bath to maintain the entire mass of the bath of a suitable density, which lies between that of the cathodic aluminum and that of the anodic alloy.

Furthermore, it has been found that the con-' sumption of electrolyte is substantially lower according to the present method, owing to the elimination of -a whole series of sources of loss. As a matter of fact, the rior processes require the use of an extra tank for the preparation and purification of the additional electrolyte with resultant lossesof electrolyte which are inherent in .the operation of that additional tank, such losses resulting from: evaporation, crust formation and impregnation of the tank All these losses are avoided by the process in accordance with this applica tion. Moreover, it is aiact, established by experience, that the direct consumption of electrolyte by evaporation is less pronounced in a tank operating in accordance with this application, with a constant quantity of electrolyte of constant composition, than in a tank operating in accordance with the prior correction methods, that is,

with a varying quantity of electrolyte of varying composition.

Besides, the method according to the invention does not at all require, contrary to some of the methods suggested up to the present time, the

feed of an electrolyte of a well determined specific' gravity.

When bringing in the salts necessary for replenishing or correcting the electrolyte, it is possible, in order to facilitate the dissolution operation, simultaneously to add salts acting as fluxes, such for instance as sodium chloride, insofar as the presence of these salts in the electrolyte has no pertunbing influence upon the electrolytic refining reaction of aluminum.

A tank for the electrolytic refining of the aluminum at 10,000 amperes and operating in accordance with the process described in the U. S. Patent No. 2,034,339 of MarchI'l, 1936, contains 1 to 2 tons of electrolyte, the normal composition of which is, for example, as. follows:

Experience has shown that the average daily consumption of such an electrolyte tank corresponds to a loss of:

Al =1420 grammes Na=1180 grammes Ba =3000 grammes Cl =3420 grammes F =2980 grammes These daily losses should be corrected by introducing into the electrolyte feed tube, the following salts:

AlF" =4400 grammes BaCl =4600 grammes NaCl =3000 grammes The partial electrolysis produced within the graphite feed tube of applicants construction purifies these salts as they dissolve in'theelectrolyte by compelling their impurities (Fe, Si, M, etc.) to pass into the aluminum left within the said feed tube. Thus, the level of the electrolyte in the tank has been reestablished and its composition rectified without in any way impeding the operation of the tanker introducing impurities.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efllcient embodiments-of the present invention, it should be well understood that I do not wish to be limited thereto as there might be changes made therein without departing from the principle of the present invention as comprehended within the scope of the appended claims.

What I claim is:

1. In the igneous electrolytic refining of aluminum in a tank containing a lower layer of an alloy of aluminum and of a heavy metal acting as anode, an intermediate layer of electrolyte and an upper layer of pure aluminum acting as cathode, a process for replenishing and correcting the electrolyte, in the refining tank itself, without interrupting the refining operation, which comprises segregating in the refining tank itself, a portion of the electrolyte, adding to said segregated part of the electrolyte salts adapted to correct the electrolyte and to maintain its composition substantially constant, and purifying the said salts by electrolyzing said segregated an alloy of aluminum and of a heavy metal acting as anode, an intermediate layer of elec trolyte and an upper layer of pure aluminumacting as cathode, a process for replenishing and correcting the electrolyte in the refining tank itself without interrupting the refining operation, which comprises segregating, in the refining tank itself, a portion of the electrolyte by introducing through the upper layer of pure aluminum, a tube of an electrically good conducting material open at both ends so that the lower edgeof said tube is immersed in the layer of electrolyte without being however in contact with the lowermost layer, removing at least a portion of the layer of pure aluminum floating on the top of the electrolyte inside said tube so as to leave a small amount of aluminum above the electrolyte segregated in the tube adding into said tube salts adapted to correct the electrolyte and to maintain its composition substantially constant and purifying the said salts by el'ectrolyzing in said tube said segregated portion or theelectrolyte while continuing the'reflning operation.

3. In the igneous electrolytic refining of aluminum in a tank containing a lower layer of an alloy of aluminum and of a heavy metal actingas anode, an intermediate layer of electrolyte and an upper layer of pure aluminum acting as cathode, a process for replenishing and correcting the electrolyte in the refining tank itself without interrupting the refining operation which comprises segregating in the refining tank itself, a portion of the electrolyte by introducing through the upper layer of pure aluminum a tube of graphite open at both ends so that the lower edge of said tube is immersed in the layer of electrolyte without being however in contact with the lowermost layer, removing at least a portion of the layerof pure aluminum floating on the top of the electrolyte inside said tube so as to leave a small amount of aluminum above the electrolyte segregated in the tube, adding into said tube salts adapted to correct the electrolyte and to maintain its composition substantially constant and purifying the said salts by electrolyzing in said tube said segregated portion of the electrolyte while continuing the refining operation.

KARL SIEDENTOPF. 

